I would say the main conflict of My Side of the Mountain is an external man vs. nature conflict. In the beginning of the book, Sam Gribley decides to live on his own out in the wilderness. He does this in a manner befitting an extreme survivalist because when he leaves civilization, he has minimal supplies. Sam doesn't take food or a lot of tools. He leaves New York in May with “a penknife, a ball of chord, an ax, and $40,” as well as flint and steel for making fires.
That is an exceptionally small amount of gear. I like camping, but Sam puts me to shame. Throughout the novel, he is continually in conflict with the nature that surrounds him. Not only does he have to deal with the challenges of finding decent shelter, but he also has to hunt and forage for food. Because his diet is so meager, Sam struggles with vitamin deficiencies and very low calorie intakes. As winter approaches, Sam discovers his meager supplies cannot adequately deal with winter conditions.
Toward the end of January I began to feel tired, and my elbows and knees were a little stiff. This worried me. I figured it was due to some vitamin I wasn't getting, but I couldn't remember which vitamin it was or even where I would find it if I could remember it.
As the book moves toward its conclusion, Sam's conflict turns into an internal conflict. Sam must decide to continue living on his own or return to his family.